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Thank you for contacting the Intel Communities.
In this case, I would like to let you know that this is an expected behavior from the system while using the HDMI 1.4b port of the NUC. This video port supports up to 4096 x 2304 @ 24Hz
The monitor may have the port but both ends are required to be HDMI 2.0 to work at 60Hz
This information can be confirmed in the page 23 of this file: http://www.intel.com/content/dam/support/us/en/documents/boardsandkits/NUC6i5SYB_NUC6i3SYB_TechProdSpec04.pdf
If any additional questions or inquiries are present, feel free to contact us back.